The present invention relates to a method for setting a shoe position in an extended-nip press and an extended-nip press.
Generally an extended-nip press comprises a press roll cooperating with a backing roll. Typically, the press roll comprises a rotating endless-loop blanket of a flexible, liquid-impervious material, a rigid and advantageously stationary roll support beam that extends axially through the interior of the endless blanket and has a stub shaft mounted at its both ends, at least one press shoe resting on the roll support beam and having a concave top face, loading means for pressing the concave top face against the flexible endless blanket so as to form a press nip zone in cooperation with the backing roll, two blanket-clamping roll heads axially movable on their respective stub shafts, clamp elements for engaging the lateral rims of the blanket to the respective roll heads and at least one element for tightening and/or moving the flexible endless blanket in the axial direction of the respective stub shaft.
The shape of the nip pressure profile generated by the press nip zone and imposed on the web passing therethrough is determined by the shape of the concave face of the press shoe and its position relative to the backing roll and the means loading the shoe. Hence, the shape of the nip pressure profile can be adjusted either by controlling the concave shape of the shoe top face or by moving the position of the shoe relative to the backing roll and/or the shoe loading means.
Among other factors, an advantageous shape of the nip pressure profile is dependent on the paper grade being manufactured. For instance, lightweight paper grades are problematic by undergoing rewetting in an extended-nip press, whereby the most advantageous nip pressure profile for these grades is adjusted such that the peak pressure in the machine direction is close to the outgoing side of the press nip zone. Thicker paper grades, thick paperboards in particular, are problematic by undergoing collapse of the web internal structure if the machine direction nip pressure profile rises excessively steeply and the maximum nip pressure is too high. Hence, thick paper grades are generally most advantageously run using a relatively smooth nip pressure profile having the peak pressure adjusted in the machine direction close to the middle of the press nip zone. Typically, a papermaking machine is used for making more than a single paper grade. Accordingly, it is desirable that the pressure profile of an extended-nip press be adjustable as required by the paper grade being manufactured.
For Instance, patent publication FI 65103 teaches the adjustment of the nip pressure profile to take place by way of providing the support means of the press shoe with transfer means adapted to shift the center of the shoe loading force relative to the shoe. In accordance with the teaching of the publication, the center of shoe loading force can be implemented in two different ways: either by using a movable support assembly adapted mechanically movable relative to the shoe or by using a stationary support assembly by means of which the magnitude of the loading force imposed on the shoe can be hydraulically varied between the leading and trailing edges of the shoe, whereby the center of the loading force is changed relative to the shoe. The arrangements disclosed in the publication are hampered by the complexity of their construction and, hence, high manufacturing costs.
Patent publication U.S. Pat. No. 4,973,384 discloses another prior-art technique of adjusting the nip pressure profile. The embodiment described in the publication has a plurality of grooves made in the cross-machine direction to the underside of the shoe. The upper end of the cylinder loading the shoe has respectively mounted thereon a cross-machine pivot pin aligned in parallel with the grooves of the shoe, whereby the pin can act as a pivotal point for the shoe. Then, the shape of the machine-direction nip pressure profile can be varied by moving the shoe position on the pivot pin of the loading cylinder from one groove to another. A disadvantage of the embodiment disclosed in the publication is that due to the substantially high forces imposed at the pivot point between the shoe of the extended-nip press and its loading cylinder, the spacing between the grooves on the shoe underside must be made relatively wide such that a sufficient portion of shoe material remains on the ridges between the grooves to bear the loading forces imposed thereon. As a result, the control of the nip loading profile takes place in rather coarse steps. Furthermore, the cylinders located underneath the shoe are subject to wear thus needing frequent maintenance.
It is an object of the method and extended-nip press according to the invention to eliminate or at least reduce the above-described problems related to the prior art.
It is a further object of the present invention to provide a method for setting the shoe position in an extended-nip press and, further, an extended-nip press, wherein the shoe position, particularly the tilt angle thereof, can be set and changed at a sufficiently high precision in order to control the nip pressure profile of the extended-nip press. It Is still a further object of the invention to provide an extended-nip press having a simple and reliable function and construction.
It is further another object of the invention to provide an extended-nip press, wherein the shoe and the element loading the same cooperate so that this combination of elements compensates for thermal expansion occurring in an extended-nip press.
To achieve the above-mentioned objects and others, the method for setting the shoe position in an extended-nip press and the extended-nip press according to the invention are principally characterized by what is stated in the characterizing parts of the appended base claims.
The method according to the present invention is characterized in that the shoe element is connected to the loading element by means of a detachable saddle element adapted between the shoe element and the loading element and that the position of the shoe element relative to the loading element is set by adjusting the relative position between the saddle element and the shoe element. In the context of the present text, saddle element refers to a preferredly planar part adapted between the loading element and the shoe element so as to connect the loading element to the shoe element in a functional manner. Loading element in the present context refers to a cylinder or an assembly acting as a loading cylinder such that the shoe element can be pressed at a desired force against a backing roll. The shoe element may comprise a single part or be assembled from a plurality of parts. To set, or change, the position, that is, the angle or alignment of the shoe element in a desired direction, the center of the force imposed by the loading element on the shoe element is changed, whereby also the shape of the nip pressure profile is altered.
In a preferred embodiment of the present invention, the surface of the saddle element facing the loading element and, respectively, the surface of the loading element facing the saddle element are shaped so that these two mating faces form a ball joint. These shaped faces may be implemented so that, e.g., the loading element face is made spherically convex while the saddle element face is made spherically concave or vice versa.
Advantageously, the saddle element and the shoe element are connected to each other by at least one dismountable keyed keyway joint. Key in the present context refers to a key part having a wedged, curved or prismatic shape that connects the saddle element to the shoe element. The key may be a separate element or, alternatively, a structural and integral part of the saddle element or the shoe element. Keyway refers to a wedged, curved or straight-walled slot suited to accommodate the insertion of the key therein for connecting the saddle element to the shoe element. In this context, keyed keyway joint refers to a joint accomplished by inserting the connecting key in the keyway made to the element to be jointed.
Particularly advantageously the surface of the saddle element facing the shoe element and/or the surface of the shoe element facing the saddle element is provided with plural keyways that are located at different distances from the center of the saddle element and of which keyways at least one is utilized for connecting the saddle element to the shoe element. Then, the position of the shoe element relative to the saddle element can be varied by using a different keyway for connecting the elements to each other.
In a preferred embodiment of the method according to the present invention, both the saddle element and the shoe element have on their mating surfaces a keyway, whereby the shoe element can be connected to the saddle element by a detachable key inserted in the keyways provided in the saddle element and the shoe element. Particularly advantageously, the key used for the connection has an asymmetrical shape, whereby the rotation of the key gives a means for changing the mutual disposition of the saddle element and the shoe element that are connected to each other by the key. Eccentricity of the key in this context refers to an asymmetrical cross section of the key relative to its center axis such that the mutual disposition of the saddle element and the shoe element connected to each other by the key can be varied depending on the position of the key.
The extended-nip press according to the present invention is characterized in that the shoe element is connected to its loading element by a detachable saddle element adapted between the shoe element and the loading element.
One of greatest benefits of the method according to the invention is that the position and, as a result, the tilt angle of the shoe can be changed in a rapid and uncomplicated fashion.
The greatest benefit of the extended-nip press according to the invention is its uncomplicated, yet extremely functional construction that can be implemented at a reasonable manufacturing cost, whereby also its maintenance and servicing becomes easy and quick. Furthermore, the construction of the extended-nip press according to the present invention is very durable in use.
An additional benefit of a preferred embodiment of the invention, wherein the saddle element and the loading element form a ball joint, is its good tolerance to thermal expansion by virtue of the joint construction that permits unidirectional tilting of the shoe element. As a result, the entire shoe element can be made from aluminum which is cost-efficient material but has a high thermal expansion coefficient. Due to the good thermal conductivity of aluminum, heat is efficiently transferred to the different parts of the shoe.
A still another benefit of the arrangement according to the invention is that the construction used therein does not need a separate support member for receiving the forces imposed on the shoe. Yet, the present construction may be complemented with limiting member as a safety precaution in malfunction situations in order to prevent the shoe element from slipping away from its normal position. During normal operation of the press, between the shoe and the limiting member remains a gap to prevent the shoe from contacting the limiting member.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.